Secondary radars, commonly designated by the initials SSR corresponding to the conventional terminology of “Secondary Surveillance Radar”, are widely used in the field of the detection of aerial targets. Secondary radars are typically fitted to fixed terrestrial platforms, and are often coupled with primary radars. Secondary radars can also be fitted to mobile terrestrial or aerial platforms. Secondary radars can also be designated according to the initials IFF, corresponding to the conventional terminology of “Identification Friend or Foe”. IFF, initially designed for discriminating between friendly or enemy targets, has subsequently branched into a plurality of modes, used notably in civil aeronautics, for detecting aircraft fitted with transponders. The transponders fitted to the aircraft emit signals in a spontaneous manner on a periodic basis, or in response to specific interrogation signals emitted by secondary radars or interrogators. The secondary radars or interrogators pick up the signals emitted by the transponders. A specific feature of the signals emitted by the transponders, termed SIF according to the acronym corresponding to the conventional terminology “Selective Identification Feature”, is that the latter take the form of non-phase-modulated pulse trains. A certain number of pulses are emitted, customarily delimited by two so-called “bracketing” pulses provided for this purpose, and commonly referred to simply as “brackets”: the absence or the presence of pulses in messages of determined duration constitutes a logical word containing certain indications specific to the aircraft, such as its identification, its altitude, etc. For example, it is possible to cite the A mode, in which the transponder of an aircraft transmits an SSR identification code, the code making it possible notably to associate, in a radar tracking system, the identification of an aircraft with a radar blip. It is also possible to cite the C mode, in which an altitude indication is added, the indication being able to for example be displayed on a control screen of an air traffic control centre, in association with the radar blip corresponding to the aircraft. In most modes considered, a transponder emits a message consisting of a sequence defined by a plurality of pulses, the pulses being emitted at an unmodulated characteristic frequency. The secondary radar detection chain then operates a decoding of the words reaching it in this form, by detecting the absence or the presence of the pulses lying between the pulses of “brackets” type, delimiting the words.
However, detection can sometimes be tricky, notably when the targets are distant and/or the signal is strongly disturbed by noise. In such cases, the signal-to-noise ratio may be very low, and require a very low detection sensitivity, thus necessitating the implementation of refined means with the aim of allowing detection of targets satisfying the regulatory constraints. Furthermore, the diversity of the targets, a greater or lesser distance from the secondary radar, requires the detection chains to cover a wide input dynamic, typically from −22 dBm to −84 dBm.
Among the technical solutions known from the prior art, a particularly advantageous technique consists in using a logarithmic amplifier, capable of covering a wide input dynamic, and of utilizing an output of the logarithmic amplifier, termed the RSSI output or “video” output, the initials RSSI standing for the expression “Received Signal Strength Intensity” (i.e. intensity of the power of the received signal). The RSSI output thus restores a signal of envelope type, representative of the reception level. The RSSI output of the logarithmic amplifier is then linked to a high-resolution analogue-digital converter for digital processing of the data.